Wavelength-division multiplexing (WDM) has enabled a dramatic increase in the transmission capacity of fiber-optic systems. WDM combines a plurality of sub-beams at different wavelength bands for propagation through an optical fiber as a multiplexed beam. As each sub-beam carries a signal, multiple signals can be transmitted simultaneously. Wavelength-selective switches (WSSs) are used to route the individual sub-beams, and the signals they carry, along particular optical paths.
With reference to FIGS. 1A and 1B, a reflective embodiment of a 1×N WSS 100A is disclosed in U.S. Pat. No. 6,707,959 to Ducellier, et al., issued on Mar. 16, 2004, which is incorporated herein by reference. The 1×N WSS 100A has a 1×N wavelength-selective switching functionality, that is, the ability to route individual sub-beams from one input port to N output ports. If used in reverse, the 1×N WSS 100A has an N×1 wavelength-selective switching functionality, that is, the ability to route individual sub-beams from N input ports to one output port.
In the 1×N WSS 100A, a multiplexed input beam is launched from an input port in a front-end unit 110 towards a spherical mirror 120. The input beam is redirected from the spherical mirror 120 to a reflective diffraction grating 130, which disperses the input beam into K sub-beams at K wavelength bands. The K sub-beams are then redirected from the reflective diffraction grating 130, via the spherical mirror 120, to a microelectromechanical system (MEMS) mirror array 140 of K mirrors, i.e. a 1×K MEMS mirror array, which routes the K sub-beams to N output ports in the front-end unit 110. Each mirror is associated with a particular wavelength band and can be tilted about a tilt axis to route the sub-beam at the particular wavelength band to one of the N output ports. The K sub-beams are redirected from the MEMS mirror array 140, via the spherical mirror 120, back to the reflective diffraction grating 130, which combines any sub-beams that are routed to a same output port. The K sub-beams are then redirected from the reflective diffraction grating 130, via the spherical mirror 120, to the N output ports in the front-end unit 110, which output the K sub-beams.
With reference to FIGS. 1C and 1D, in a transmissive embodiment of the 1×N WSS 100B, the optical path of the 1×N WSS 100A is “unfolded” by replacing reflective components by transmissive components. In these simplified drawings, the front-end unit 110 is omitted, and an input port 111 and N output ports 112, e.g. N=5, are illustrated individually instead. The spherical mirror 120 is replaced by a first spherical lens 121 and a second spherical lens 122, and the first and last reflections from the spherical mirror 120 are omitted. The reflective diffraction grating 130 is replaced by a first transmissive diffraction grating 131 and a second transmissive diffraction grating 132.
In the 1×N WSS 100B, a multiplexed input beam is launched from the input port 111 towards the first transmissive diffraction grating 131, which disperses the input beam into K sub-beams, e.g. K=3, at K wavelength bands. The K sub-beams are then redirected from the first transmissive diffraction grating 131, via the first spherical lens 121, to the MEMS mirror array 140 of K mirrors, which routes the K sub-beams to the N output ports 112. Each mirror is associated with a particular wavelength band and can be tilted about a tilt axis to route the sub-beam at the particular wavelength band to one of the N output ports 112. The K sub-beams are redirected from the MEMS mirror array 140, via the second spherical lens 122, to the second transmissive diffraction grating 132, which combines any sub-beams that are routed to a same output port 112. The K sub-beams are then received by the N output ports 112, which output the K sub-beams.
Other examples of 1×N WSSs including a single MEMS mirror array are disclosed in U.S. Pat. No. 6,498,872 to Bouevitch, et al., issued on Dec. 24, 2002, in U.S. Pat. No. 6,760,501 to Iyer, et al., issued on Jul. 6, 2004, and in U.S. Pat. No. 6,810,169 to Bouevitch, issued on Oct. 26, 2004, all of which are incorporated herein by reference. A 1×N WSS including a plurality of MEMS mirror arrays is disclosed in U.S. Patent Application Publication No. 2011/0170867 to Keyworth, et al., published on Jul. 14, 2011, which is also incorporated herein by reference.
Two 1×N WSSs, one of which is used in reverse, may be cascaded to obtain an M×N switching functionality, that is, the ability to route individual sub-beams from M input ports to N output ports. With reference to FIG. 2, by cascading an M×1 WSS 201, which has M input ports 211, e.g. M=4, and one output port 212, with a 1×N WSS 202, which has one input port 213 and N output ports 214, e.g. N=5, an M×N switching cascade 200 is obtained. An example of a hitless M×N switching cascade is disclosed in U.S. Patent Application Publication No. 2010/0061727 to Colbourne, et al., published on Mar. 11, 2010, which is incorporated herein by reference.
However, in such M×N switching cascades, the sub-beams must be combined by a diffraction grating upon exiting the M×1 WSS, coupled into an optical fiber between the M×1 WSS and the 1×N WSS, and re-dispersed by a diffraction grating upon entering the 1×N WSS. To avoid the losses associated with these two diffraction-grating passes and the optical-fiber coupling, an M×N WSS is desired.
Most prior-art M×N WSSs, such as those disclosed in U.S. Pat. No. 6,711,316 to Ducellier, issued on Mar. 23, 2004, in U.S. Pat. No. 6,694,073 to Golub, et al., issued on Feb. 17, 2004, in U.S. Pat. No. 7,106,926 to Cerato, issued on Sep. 12, 2006, and in U.S. Patent Application Publication No. 2010/0172646 to Colbourne, published on Jul. 8, 2010, all of which are incorporated herein by reference, include an M×K MEMS mirror array and/or a K×N MEMS mirror array. Unfortunately, such large MEMS mirror arrays are expensive and difficult to fabricate.
Examples of M×N WSSs including two 1×K MEMS mirror arrays are disclosed in U.S. Pat. No. 7,720,329 to Presley, et al., issued on May 18, 2010, and in U.S. Pat. No. 7,769,255 to Nagy, et al., issued on Aug. 3, 2010, which are incorporated herein by reference. However, these M×N WSSs do not include a relaying element having optical power between the two 1×K MEMS mirror arrays for re-imaging the sub-beams. Therefore, the sub-beams must have a large enough beam size to remain collimated while propagating between the two 1×K MEMS mirror arrays, resulting in the M×N WSSs being undesirably large.